1. Field of Invention
The identification and quantification of a chemical vapor in a vapor mixture by the physical parameters of a sensor which change upon exposure to the vapor mixture.
2. Prior Art
[1] W. Patrick Garey, Bruce R. Kowalski, "Chemical Piezoelectric Sensor and Sensor Array Characterization", Anal. Chem. 1986, 58, 3077-3084.
[2] Susan L. Rose-Pehrsson, Jay W. Grate, David S. Ballantine, Jr., Peter C. Juts, "Detection of Hazardous vapors Including Mixtures Using Pattern Recognition Analysis of Responses from Surface Acoustic Wave Devices", Anal Chem 1988, 60, 2801-2811.
[3] Pyke et al., "Apparatus and Method for Early Detection and Identification of Dilute Chemical Vapors", U.S. Pat. No. 4,895,017, Jan. 23, 1990.
This invention relates to an apparatus and method for sensing and identifying the chemical vapor and resolving a chemical composition of a vapor mixture and is specifically directed to an apparatus and method which uses the physical parameter changes of a sensor when exposed to the vapor mixture to identify a particular compound in the chemical vapor mixture, and to determine concentration of one or more components in the mixture. This invention includes a method for discriminating instrument responses to chemical vapors from instrument noise and chemical interferant noise sources.
This invention will be explained in connection with the use of Surface Acoustic Wave (SAW) sensors but it should be understood that this invention may be used with any sensor which will react upon exposure to a chemical vapor.
While some of the definitions hereinbelow are prior art, for the purposes of facilitating the description of this invention, the following definitions will apply:
SAW sensor--a pair of SAW oscillators (SAWs). One, called sample SAW is coated with a selected coating material and periodically exposed to the sample air and another, called reference SAW, is not coated or, if coated, is not exposed. PA1 SAW signal--one cycle difference in frequency oscillation between a sample and a reference SAW, i.e., the measurement of oscillation frequencies as the SAWs are first exposed to clean air, then subjected to a contaminated atmosphere, and finally re-exposed to clean air. PA1 SAW array--a set of several SAW sensors with different coatings on the sample SAWs or a set of several SAW sensors having the same coating or different coatings or any combination of coatings on the sample SAWs with each SAW sensor held at the same temperature or at different temperatures from the others. PA1 Compound--a chemical that can produce a non-negligible signal on a SAW sensor. PA1 Target Compounds--those chemicals whose presence or concentrations need to be determined in an air stream, and to which a selected SAW sensor is designed to be responsive. PA1 Contaminants--compounds other than target compounds present in the sample air stream, also called interferants. PA1 Mixture--all the compounds contained in the sampled air stream. PA1 Noise--a signal observed when the sample air stream and a reference air stream are the same air. Also, a signal, either instrumental or caused by an interferant in the sampled air, that may be interpreted as a target compound response. PA1 Sensitivity--Signal-to-noise ratio at a signal produced by a unit concentration of a compound of interest. For the fingerprint method, it is an amplitude of the steady state part of a signal produced by a unit concentration. PA1 Characteristic transient, characteristic response of a chemical--noise free, one cycle signal (or any part of thereof) produced by a unit concentration of this chemical at a given SAW. PA1 Characteristic spectrum of a chemical--a vector consisting of the characteristic transients from all the sensors of a SAW array. PA1 a. Instead of a one data point per cycle, this method uses a vector which represents the whole cycle or a part thereof. Therefore, a mixture can be separated by using fewer SAW sensors since the method increases dimensionality of the sampling space, PA1 b. There is no need to wait for a steady state response, and PA1 c. There are much milder requirements on selectivity of the individual SAW sensors since there are more degrees of freedom in the data obtained with the fingerprint method which has one degree of freedom per SAW sensor. PA1 a. does not use any parametric model for the physical phenomena, PA1 b. uses the whole cycle or any part of it instead of a loading phase only, PA1 c. uses a characteristic spectrum of a mixture which is a linear combination of the characteristic spectra of the individual components while a time constant of a mixture is not a linear combination of the individual time constants, PA1 d. does not require a high selectivity of the SAW coating, PA1 e. provides a number of degrees of freedom per SAW and is not predetermined to 2 but can be determined from the available data, e.g., via Singular Value Decomposition of a calibration matrix X, PA1 f. complicated mixtures can be separated using a fewer number of SAW sensors, and PA1 g. there is no need for a parameter estimation procedure.
It is recognized that a SAW measures air concentrations of a particular compound by changing its resonant frequency in the presence of additional mass on the coated SAW surface. The target compound sorbs into a selective coating on the surface of the SAW, equilibrating the activity (or concentration) of the compound in the coating film with the activity (or partial pressure) of the target compound in the air. The resulting frequency shift is proportional to the added surface mass density at a steady state equilibrium [1]. To allow multiple measurements, the coating material needs to have a reversible response, namely, after a target compound is removed from the air stream, the frequency shift (between the coated and uncoated SAWs) should return to zero (only with the baseline removed) as the target compound desorbs from the SAW coating. Therefore, SAW sensor measurements are done in cycles where a SAW sensor is exposed alternately to analyzed and reference (nominally uncontaminated) air streams. A typical SAW sensor output signal is shown in FIG. 4.
Each measurement cycle, as shown in FIG. 6, (which is one of the measurement signals in FIGS. 4 and 5) has a loading phase (curve A) when a compound penetrates into the coating, a steady state phase (curve B) when compound concentration in the coating reaches equilibrium with t e compound concentration in the air above the coating, and an unloading phase (curve C) during which the chemical is washed out of the surface coating by a stream of reference air. The loading and unloading phases are present in any cycle, but a steady state phase sometimes is not reached because the exposure time of the SAW coating to a chemical is too short compared to the mass transport and diffusion rates for that chemical in the system.
Different coatings may have different sensitivities toward different compounds. When it is possible to find a set of coating materials with very high selectivity toward particular compounds, then a SAW array comprising more than one SAW sensor with different coatings on the sample SAW of each SAW sensor can resolve a mixture of several compounds. The normal arrangement of a SAW array is to have almost as many SAW sensors as there are different compounds in the mixture with each sample SAW possessing a different coating with a high selectivity toward one of the compounds in the mixture.
It is to be understood that a SAW array may be a set of more than one SAW sensor, each with the same coating, or different coatings, or a combination of coatings on the sample SAWs, and which are held at the same temperature or at different temperatures so as to be sensitive to each of the compounds of interest. The steady state response of a SAW array to each of the compounds of interest is called the compound's "fingerprint" on this particular SAW array.
Currently, there are two major methods for processing SAW data.
A. Fingerprint Method: one data point per SAW
The existing SAW sensors use a steady state frequency shift of one measurement cycle (curve B in FIG. 6). Then a maximum of frequency shift (FIG. 7), or an integral of a one cycle frequency shift (FIG. 8), is used. This produces one data point per SAW sensor per measurement cycle.
The current state of the art uses N-dimensional cluster analysis to separate target compound signals from those signals due to contaminants or interferants [2]. By significant computational efforts, it is often possible to increase the number of possible contaminants (using the same number of SAW sensors) by 20% to 30%. Usually, several similar contaminants are grouped into one class by observing that their "fingerprints" occupy the same region in an N-dimensional space.
The cluster analysis method is computationally expensive and does not give significant savings in the number of SAW sensors needed for multicomponent separation.
Also, the method discriminates compounds based on their steady state responses on the SAW array. This need for a steady state response requires a measurement time which is long, especially for small concentrations of the components of interest.
B. Analysis based on Time Constant Determination
An alternative method [3], instead of relying exclusively on the steady state (static) characteristic, tries to make use of some dynamic information. The method introduces one more parameter--the target compound sorption time constant .tau.. This method relies on an explicit sorption model ##EQU1## where M.sub.o is the mass sorbed at a steady state, and R is a constant depending on a rate of air flow. In this case, each SAW sensor produces two data points that are characteristic for a given compound and its concentration: diffusion time constant .tau. and a steady state sorption M.sub.o. Since an explicit form of an sorption transient is used, both characteristic constants .tau. and M.sub.o can be determined before a steady state is achieved. This allows a reduction in measurement time and provides an early determination of a target compound concentration before a steady state response is reached. In addition, the sorption time constant gives an extra degree of freedom to determine which component is present in the air stream.
However, the time constant method requires the SAW coatings to have a very selected sensitivity to the target compounds. When several compounds are present, equation (1) becomes a sum of several exponents: ##EQU2##
Equation (2) has 3.times.K unknown parameters: M.sub.ok, R.sub.k, .tau..sub.k : k=l . . . K; where K is the number of different compounds in the air stream. M (t), the mass of analyts sorbed at time t is nonlinear with respect to unknown parameters and becomes unmanageable for large K. Since the instrument response is to first order linear with sorbed analyt mass, the complex behavior of mixtures leads to unmanageable data analysis.